TERRESTRIAL VISITORS

Strange Cloud Explained

March 2, 2015

Question: I was out on my deck last night, enjoying the sunset, and I saw this really strange cloud with what looked like evenly-spaced “fingers” along one edge. I never saw anything like that before. Unfortunately, I didn’t have my camera so I can’t send you a photo. Do you have any idea what I saw? — PS, San Francisco, CA

Answer: Well, without a photo, and just going by your description, what I suspect you saw was something called a Kelvin-Helmholtz cloud. It was formed by a Kelvin-Helmholtz wave. K-H waves often arise in moving fluids (liquid or gas). When air blows over water, it’s what causes water waves. When two layers of air blow past each other, a K-H cloud can be formed by the same process. And they can indeed look like “waves.” Several great examples can be seen here.

K-H clouds are common in areas with strong prevailing winds, like coastlines. In other places, like Arizona (where the above photo was taken), they are less common. The photo shows the view to the southwest. In this case, there was a front advancing from the west, and an upper level wind (yellow arrow) blowing roughly perpendicular to the motion of the front. Where the two air masses meet, shear occurs.

Shear is a distorting force caused by the relative motion of two things in physical contact. For solid bodies, it’s mediated by simple friction and causes flexure. For fluids, it’s mediated by diffusion and turbulence, and can cause mixing, waves, and vortices. The “fingers” on the cloud are formed as K-H waves compress, then expand, water vapor in the atmosphere. Those contrails you often see behind jets are caused by a similar compression-expansion cycle.

When K-H waves occur in dry air they are invisible to the eye. K-H clouds will form only if the air is close to its dew point. Note that the “cloud fingers” in the photo are not the K-H waves themselves — they’re a result of K-H waves.

The regular periodic spacing of the “fingers” is a little harder to understand, but an analogy to water waves might help. You’ve no doubt noticed, from the shore or the air, how waves in water also show regular periodic spacing. In both cases, the mechanism is the same: As a wave begins to grow from a small random instability, it draws energy from adjoining regions in the fluid. This limits how close together two waves can form. Wherever enough energy becomes available, the next wave will start forming. And so it goes. Periodicity often emerges from random processes in Nature.

If you have difficulty understanding K-H waves, check out this outstanding video by the University of Cambridge. It runs two minutes and shows periodic K-H waves forming between two moving layers of colored liquid.

I get a lot of questions about “strange” clouds. If you search my archive you’ll see many examples. In every case, there’s a reason they look the way they do. Having a background in science makes it easier to understand clouds, but simple curiosity is more important. Whenever I see an unusual cloud I find myself asking “Why?” When I can’t figure it out, the answer is almost always available online.